MicroRNA-34/449 controls mitotic spindle orientation during mammalian cortex development

Correct orientation of the mitotic spindle determines the plane of cellular cleavage and is crucial for organ development. In the developing cerebral cortex, spindle orientation defects result in severe neurodevelopmental disorders, but the precise mechanisms that control this important event are not fully understood. Here, we use a combination of high‐content screening and mouse genetics to identify the miR‐34/449 family as key regulators of mitotic spindle orientation in the developing cerebral cortex. By screening through all cortically expressed miRNAs in HeLa cells, we show that several members of the miR‐34/449 family control mitotic duration and spindle rotation. Analysis of miR‐34/449 knockout (KO) mouse embryos demonstrates significant spindle misorientation phenotypes in cortical progenitors, resulting in an excess of radial glia cells at the expense of intermediate progenitors and a significant delay in neurogenesis. We identify the junction adhesion molecule‐A (JAM‐A) as a key target for miR‐34/449 in the developing cortex that might be responsible for those defects. Our data indicate that miRNA‐dependent regulation of mitotic spindle orientation is crucial for cell fate specification during mammalian neurogenesis. Synopsis A high‐throughput imaging screen identifies a set of miRNAs that control mitotic spindle orientation in neuronal progenitors and radial glial cells and whose expression is required for normal cortical development in mouse. An in vitro screen identified miR‐34/449 as a candidate cortical miRNA involved in mitotic spindle orientation. miR‐34/449 is required for normal neurogenesis during mouse cortical development. The production of neurogenic intermediate progenitors depends on miR‐34/449, which controls mitotic spindle orientation in radial glial cells. miR‐34/449 regulates spindle orientation at least in part by directly targeting and inhibiting JAM‐A. Graphical Abstract A high‐throughput imaging screen identifies a set of miRNAs that control mitotic spindle orientation in neuronal progenitors and radial glial cells and whose expression is required for normal cortical development in mouse.